The use of humic acid and fractions thereof as dyes is well known. ("Recent Progress in the Chemistry of Natural and Synthetic Coloring Matters," T. S. Gore et al. eds., Academic Press, N.Y. 1962, pps. 99-112.) Recently, a variety of cigarettes manufactured with brown paper wrappers have enjoyed increased popularity among smokers, and some of these cigarettes are fabricated using brown wrappers stained with humic acid. Processes for staining cigarette paper brown have included the use of dyes such as synthetic azo dyes, dyes produced from walnut shells, caramel, tannic acid and the like. However, the aforementioned stains or dyes are undesirable for a variety of reasons. First, the azo dyes, which contain large amounts of nitrogen, may produce undesirable pyrolysis products and, therefore, could be unsuitable for use in smoking articles. Secondly, the amount of caramel required to produce the desired intensity of brown color considerably inhibits the burn rate of the paper. The use of such stains as tannic acid may provide the desired brown color; however, treatment of this type simultaneously reduces the porosity of the paper thereby also reducing the rate of combustion. In addition, adverse affects on the gas phase composition are also noted (see Austrian Pat. No. 175,148).
By definition, humic acids are allomelanins found in soil, peat, and low-rank coal. They are generally alkali soluble and precipitated in the presence of acids. From a chemical standpoint, humic acids generally consist of a mixture of complex macromolecules characterized as having polymeric phenolic structures with the ability to chelate with metals. In addition, humic acids have a strong base-binding power, and this ion-exchange capability can be used advantageously in their use as dyes. There are many variations of humic acid depending upon differences in the plant remains from which they originate as well as the soil, climate, microflora, drainage, etc.
Humic acids, by nature, are intensely chocolate brown in color; and because of their natural origin, they are particularly preferred over synthetic dyes as staining pigments for producing brown wrappers or papers for smoking products. Generally, an alkaline solution is prepared by mixing the powdery humic acid with an alkali metal hydroxide, i.e., sodium hydroxide, to form a soluble humate salt. This solution is used to impregnate the paper on one or both sides, and this is followed by a fixing step, generally with salts such as aluminum, calcium, iron, chromium and the like. In essence, sodium ions are partially exchanged for the other metal ions added in the fixing process, thus leaving a water insoluble humate salt on the paper.
henning in Allgemeine Papier-Rundschau, No. 31:1027 (21 August 1967) describes methods for staining paper, and especially paper suitable for cigarette wrappers, with Sap Brown (also termed "nut stain" or humates) at about neutral pH. The Sap Brown may be rendered insoluble on the paper by fixing with aluminum or iron sulfate. We have found that certain disadvantages are encountered when utilizing the foregoing method of Henning. For instance, when aluminum sulfate is employed as the fixing agent, an undesirable white masking of the rich brown color is observed. When iron sulfate is used as the fixing agent, and the paper is ultimately used for smoking products, a brown ash forms on smoking. It is generally recognized that a grey to white ash is more preferable, particularly from an appearance standpoint.
German Pat. No. 957,361 discloses dyeing methods for yellow straw with an alkali humate solution. The humate is fixed on the straw by means of iron or chromium salts such as, for example, FeSO.sub.4.7H.sub.2 O or [CrCl.sub.2 (H.sub.2 O).sub.4 ]Cl-2(H.sub.2 O). Iron is unsuitable for the aformentioned reasons. The use of chromium salts in smoking products would be undersirable because of their well established toxicity. See, for example Dangerous Properties of Industrial Materials, N. Irving Sax, Fourth Edition, 1975 pages 558-9.
Others have suggested that various cations are useful for precipitating humates, and they include lead, copper, calcium, potassium, and the like. However, none have suggested that the cations mentioned would be suitable for use as a fixing agent when staining paper for ultimate use in smoking articles. Moreover, no suggestion has been made to indicate that a particular cation, magnesium when used as a fixing agent, might be preferable for use in smoking articles due to improved smoking characteristics, improved appearance, or lowered gas phase constituents on smoking.
Austrian Pat. No. 175148 to Ringer discloses the use of various acids in combination with cigarette paper to effect a denicotinization of the smoke. The porosity of the paper is decreased by Ringer's method, and the addition of salts, such as magnesium sulfate, apparently reduced the porosity to an even greater extent thereby resulting in a more significant reduction of nicotine in the smoke.
We have observed that a post-treatment of humate stained paper with magnesium sulfate has little effect on nicotine delivery or reduced porosity. These observations will be described in detail hereinbelow.
Analytical studies using humic acid-treated brown wrappers have indicated in some instances a tendency toward increased gas phase constituents. Various attempts have been made to reduce constituents, such as carbon monoxide, by using more efficient filter elements, by increasing the degree of ventilation in filters, or by increasing the porosity of the paper or wrapper, etc. However, none of these methods has proven to be entirely satisfactory.